You need to be looking at the polyurethanes, its common to use a lose some what sacrificial buffer pad too between the metal and the pressure side diaphragm to deal with those edges, also common to not use oil as the working fluid! Large - deep profiles will often have both shaped mats and diaphragms.
Pressure needed is directly dependant on what your doing with regard to thickness, not just material, if you want to do 3mm aluminum, its going to be able to do 1mm steel etc, hence theres some cross over. Pressure also varies greatly with the shape your trying to make sharper details need more pressure theres no free ride there.
Key thing to remember, a press in the hydro-forming world will often have a active bed in the hundred of square inches range, at 10Kpsi, ie 5 tons a square inch your into some seriously large structures very very very fast to contain the total generated forces safely.
Free forming does not work as well as most people think it does, metal has grain and how it stretches is not like a perfect rubbery skin but i know a lot of the aircraft stuff simply uses hard wood tooling, the tooling only has to support the working force and really need not be all the clever.
